Can not edit only a part of my object in edit mode blender: 2.90.1
while you may think there is one single object, I suspect there are child objects, expecially since your object is named Cylinder.002 (implying there might be other Cylinder objects.  Try expanding the Cylinder.002 object to reveal the children.

Be sure to exit the Edit mode of the parent before trying to select another object.
Also, you can recover File: Recover from a previous session or Auto Save.